People should not rely on that to any degree more than they would rely on colocated processes on a VM being isolated. The easiest way to be safe is to assume that all containers are already broken out of - what would you do then? Make sure processes are running as non-root, use various protection layers (pick your poison - SELinux, gresecurity, etc.), take away capabilities, and don't run workloads you don't trust.
sure, any Xen guest escape receives equal amounts of press for exactly that reason: It's an isolation barrier breaking down. However, trivial exploits breaking VM isolation have been relatively rare lately.
